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Could be a faulty T-Voice handset. Have you tried another handset? If its paired to a Telstra Gateway does the call also drop out when using a phone connected to green phone port on the Gateway?
DJN2130 is the Telstra Gateway frontier. It could be modem's DECT base is faulty. Do you have a normal phone you can plug into one of the green ports on the modem to check if that has the same problem.